Mahaney wrote a note showing US share of time spent online at Facebook, Google sites, Yahoo sites, Microsoft sites, and AOL. The accompanying chart, which you can see above, shows a steep rise for Facebook compared to a slower rise for Google, a relatively flat line for Microsoft, a decline for Yahoo, and a serious drop for AOL.

As people spend more time on Social Media sites, it would be logical to assume that they would do more Search activity on these sites. Use of portal sites and direct entry (to Websites) appear to have declined as a means to Search for content. Today, most of the Searches done on Facebook are “people” searches, but as Facebook increasingly socializes content and commerce, we would expect people to find rich Search results influenced by social signals from their friends.

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Social networks predominantly earn revenue via Online Display advertising and a majority of the ad spend comes from self service (i.e. advertisers posting ads directly themselves). Inventory remains relatively inexpensive; engagement time is high, and therefore inventory levels are significant. This is reflected in the growth in share of Ad impressions in the US over a year from little over 20% at the end of 2009 to 34% at the end of 2010. More specifically, per comScore, Facebook was number 1 based on Ad impressions in 2010, and delivered almost double the number of Ad impressions as the next closest player.

In Q1 2008, Facebook only occupied two percent of a typical American's day. In Q3 2011, the social network takes up eight times more of your typical 24 hours: around 16 percent. Even though Facebook is still a private company, the site already dominates the Internet in a way that makes it impossible for investors to ignore.

Facebook may have started out as a fun distraction, but it has become integral to many people's daily routines. The graph shows that at least for Americans, the service is becoming more and more essential.